1. Field of the Invention
This invention relates to semiconductor light emitting device and a method of fabricating the same, in particular, a process for producing semiconductor lasers. The device and the process according to the present invention are usable appropriately in semiconductor lasers which should have high light output power and high reliability, for example, excitation light sources for optical fiber amplifier and light sources for optical data storage system. Moreover, the device and the process of the present invention are applicable to LED of super-luminescent diodes, etc. wherein the facet of the light emitting device serves the light emission vertical cavity surface emitting lasers, etc.
2. Description of the Related Art
In recent years, optical data processing technology and optical communication technology have achieved brilliant extraordinary results exemplified by high-density recording with the use of optical magnetic discs and two way communication with optical fiber networks.
In the communication technology, for example, studies have been energetically made in various fields to develop large-capacity optical fiber transmitters usable in the coming multimedia age as well as Er3+-doped optical fiber amplifiers (EDFA) as signal amplifiers flexibly applicable to these transmission systems. Under these circumstances, it has been required to develop semiconductor lasers with high output power and high reliability which are essentially required as a component of EDFA.
The emission wavelengths usable in EDFA theoretically include the following three wavelengths, i.e., 800 nm, 980 nm and 1480 nm. By taking the characteristics of amplifiers into consideration, it is known that excitation at 980 nm, among all, is most desirable from the viewpoints of amplifier efficiency, noise figure, etc. It is needed that lasers with the emission wavelength of 980 nm have two contrary characteristics of high output and high reliability. Moreover, there are demands for lasers with wavelength in the vicinities thereof (for example, 890-1150 nm) as light source of secondary harmonic generation (SHG) and a source of thermal laser printers. In addition thereto, it has been urgently required to develop highly reliable lasers with high output in various fields.
In the field of data processing, attempts have been made to increase the output and shorten the wavelength of semiconductor lasers to achieve high-density recording and rapid writing and reading. That is to say, it has been strongly required to increase the output of laser diodes (hereinafter referred to simply as LDs) with the conventional emission wavelength of 780 nm and studies have been energetically made to develop LDs of 630 to 680 nm.
To achieve both of high output and high reliability which are essentially required in these lasers, there have been proposed a number of methods, for example, one comprising making the band gap in the active layer region around the facets so as to suppress the light absorption in the vicinities of the facets. Lasers with these structures, which are generally called window-structure lasers or non absorbing mirror (NAM)-structure lasers, are highly effective in establishing high output laser diodes.
On the other hand, JP-A-3-101183 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationsxe2x80x9d) proposes another method for solving the above problem. According to this patent, it is effective to form a contamination-free facet and then form a passivation layer or a part of the same with the use of a material which undergoes neither any reaction with the semiconductor facet nor diffusion per se and contains no oxygen.
As known reference similar to the above patent JP-A-3-101183, citation may be made of L. W. Tu et al., In-vacuum cleaving and coating of semiconductor laser facets using Si and a dielectric, J. Appl. Phys. 80 (11) Dec. 1, 1996. According to this paper, when cleavage is performed in vacuum in the step of coating laser facets with an Si/AlOx structure, the carrier recombination in the cleaved facet is retarded and thus the initial catastrophic optical damage (COD) level is increased.
Further, there has been known a technique for inserting an Si layer having an optical thickness corresponding to xc2xc of the emission wavelength between a coating film and each semiconductor layer so as to displace the facets from the anti-node of the standing wave in the direction of the cavity, thus lowering the electric field strength at the beam emission facet.
For example, there have been already developed semiconductor lasers of 980 nm or therearound and withstanding continuous use for about 2 years at light output of 50 to 100 mW and a process for producing the same. When operated at higher light outputs, however, these lasers are rapidly degraded, thus showing poor reliability. The same applies to LDs of 780 nm or 630-680 nm. Thus, it is the problem which now confronts all semiconductor lasers, in particular, those with the use of GaAs substrates to ensure a high reliability at higher output.
One of the reasons therefor resides in the degradation of the diode facet exposed to extremely high light output density. As well known regarding GaAs/AlGaAs semiconductor lasers, there are a number of surface states (gap state) in the vicinities of facet. Since these states absorb the output light, the temperature in the vicinities of the facets is generally higher than the temperature at bulk of the laser. This increase in temperature further narrows the band gap in the vicinities of the facets and then the output light can be more easily absorbed, thus causing a positive feedback. This phenomenon is known as so-called COD observed when a large current is injected instantly. On the other hand, there arises a problem, in common to a number of semiconductor laser elements, of the sudden failure associating a decrease in the COD level after long time driving. Although attempts have been vigorously made to solve these problems as described above, the technical level at the present stage is insufficient.
An LD with the window-structure can be produced by, for example, epitaxially growing a semiconductor material transparent to the emission wavelength on the laser facets. In this method, the epitaxial growth is performed on the facets while making the laser in the so-called bar state, which makes the subsequent electrode step highly troublesome.
Furthermore, there are various methods which comprise intentionally thermal-diffusing or ion-implanting Zn, Si, etc. as impurities into an active layer in the vicinities of laser facets so as to disorder the above-mentioned active layer, as proposed by JP-A-2-45992, JP-A-3-31083 and JP-A-6-302906.
During the production of an LD, impurities generally diffuse in the epitaxial growth direction of the laser element toward the substrate. Accordingly, there arise problems in controlling the diffusion depth and controlling the horizontal diffusion to the cavity direction, which makes stable production difficult.
When ion-implantation is carried out, ions with high energy are introduced from the facets. As a result, damages frequently remain on the LD facets even after annealing. Moreover, there arise another problem that an increase in the reactive current accompanying the decrease in the resistance in the region into which impurities have been introduced would increase the threshold current and driving current.
On the other hand, the process disclosed in JP-A-3-101183 as cited above, which comprises forming a contamination-free facet and then forming a passivation layer or a part of the same with the use of a material which undergoes neither any reaction with the semiconductor facet nor diffusion per se and contains no oxygen, suffers from technical problems as will be described below.
It is generally impossible to prevent the formation of non-radiative recombination centers such as Gaxe2x80x94O and Asxe2x80x94O on the facet at cleavage by performing the operation in the atmosphere in, for example, a clean room. From this point of view, it is essentially required to form a passivation layer in situ at the point of cleaving for the xe2x80x9cmethod of forming a contamination-free facetxe2x80x9d as disclosed in Claim 1 in the gazette of this patent. To embody this method in practice, the cleavage should be carried out in vacuum as stated in Claim 10 in the gazette. For an effect cleavage in vacuum, an extremely complicated procedure and troublesome labor are required in general, compared with the case where cleavage is effected in the atmosphere. Many non-reactive recombinatiori centers are formed on facets formed by dry-etching as stated in Claims 11 to 14 in the gazette, compared with facets formed by cleavage. Thus, this dry etching procedure is unsuitable for the production of LDs which should have high reliability.
The optimum examples of the passivation layer are Si (single crystal or polycrystal Si) and amorphous Si. However, there is no substance never undergoing diffusion in general. In semiconductor lasers which are to be operated at high output and high temperature for a long time, in particular, it is feared that the passivation materials disclosed in the above patent might diffuse.
Although it is described in L. W. Tu et al., In-vacuum cleavage and coating of semiconductor laser facets using Si and a dielectric, J. Appl. Phys. 80 (11) Dec. 1, 1996 as cited above that when an Si/AlOx structure is cleaved in vacuum in the step of coating onto a laser facet, the carrier recombination on the cleaved facet is retarded and thus the initial COD level is increased. However, this reference refers to neither reliability over a long time nor the relationship between coating and the LD structure.
Further, there has been known a technique for inserting an Si layer having an optical thickness corresponding to xc2xc of the emission wavelength of between a coating film and each semiconductor layer so as to displace the facets from the anti-node of the standing wave in the direction of the cavity, thus lowering the electric field strength at the beam emission facet. However, this technique suffers from a fear that Si per se would serve as a light absorption in the emission wavelength region embodied by usual semiconductor lasers, in particular within the range of from 400 to 1600 nm needed for high-output LDs and thus there is a possibility that the degradation of devices might be accelerated by an increase in temperature on facets.
An object of the present invention, which has been completed to solve the above problems, is to provide semiconductor lasers capable of suppressing the surface state density on the facets of semiconductor light emitting devices such as semiconductor lasers for a long time and stable operating even when the passivation layer diffuses, and a process for conveniently producing the same. In other words, the present invention provides high-performance semiconductor lasers establishing both of high output and high reliability by preventing degradation on facets.
The present inventors have found out that semiconductor light emitting devices having compound semiconductor layers containing at least a first conduction type of clad layer, an active layer transparent to the emission wavelength in the vicinities of the facets and a second conduction type of clad layer formed on a substrate and having cavity facets coated with passivation layers are much superior in high output and high reliability to the conventional ones, thus completing the present invention.
Accordingly, the gist of the present invention resides in a compound semiconductor liglit emitting device having a compound semiconductor layer containing at least a first conduction type of clad layer, an active layer and a second conduction type of clad layer formed on a substrate and have a cavity facets, characterized in that the active layer is transparent to the emission wavelength in the vicinities of the facets, preferably free from oxide, and that the facets are coated with a passivation layer.
The present inventors have also found out that semiconductor light emitting devices having a p-type active layer preferably containing In and having a cavity facet coated with a passivation layer containing Si, more preferably having disordered cavity facet free from oxide are much superior in high output and high reliability.
Accordingly, the gist of the present invention resides in compound semiconductor light emitting devices wherein a first conduction type of clad layer, an active layer and a second conduction type of clad layer are grown on a substrate and two facets opposite to each other form a cavity, characterized in that said conduction type of active layer is P and that the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets are each coated with a passivation layer containing Si.
The present inventors have further conducted extensive studies to solve the above-mentioned problem. As a result, they have found out that when said facets are treated by irradiating with plasma having energy falling within an optimized range from the facet side, the facets can be easily made transparent without any problems (for example, control of the diffusion depth and the horizontal diffusion to the cavity as observed in the case where impurities are diffused; difficulties in the electrode formation as observed in the case where epitaxial growth is performed on the facets; passage of reactive current accompanying the decrease in the resistance in the vicinities of the facets), thus giving semiconductor lasers achieving both of high output and high reliability. The present invention has been completed on the basis of this finding.
Accordingly, the gist of the present invention resides in a process for producing a semiconductor laser having a first conduction type of clad layer, an active layer and a second conduction type of clad layer formed on a semiconductor substrate, characterized in that at least one facet forming a cavity is irradiated with plasma having energy of from 25 eV to 300 eV.
The present inventors have further conducted extensive studies to solve the above-mentioned problem. As a result, they have found out that semiconductor light emitting devices produced by a process for producing a semiconductor light emitting device having a compound semiconductor layer containing at least a first conduction type of clad layer, an active layer and a second conduction type of clad layer formed on a substrate and having a cavity, which comprises forming the compound semiconductor layer on the substrate by crystal growth; next forming the cavity facets; then desorping a part of the elements constituting at least the active layer in the vicinities of the facets exposed on at least one of the facets via irradiation with ion, electron, heat and/or light, etc. to thereby form a region transparent to the emission wavelength in the vicinities of the semiconductor light emitting device; and forming a passivation layer in vacuum; are much superior both in high output and high reliability to the conventional ones, though the production process is highly convenient, thus completing the present invention.
Accordingly, the gist of the present invention resides in a process for producing a semiconductor light emitting device having a first conduction type of clad layer, an active layer and a second conduction type of clad layer formed on a substrate and having a cavity, characterized by comprising forming the compound semiconductor layer on the substrate by crystal growth; next forming the cavity facets; then desorption of a part of the elements constituting at least the active layer in the vicinities of the facets exposed on at least one of the facets; and forming a passivation layer in vacuum.
The present inventors have further conducted extensive studies to solve the above-mentioned problem. As a result, they have found out that, in a semiconductor laser having a first conduction type of clad layer, an active layer containing quantum well and a second conduction type of clad layer on a semiconductor substrate, elements with high vapor pressure in the vicinities of the facets can be selectively desorbed by irradiating ion, electron, light and/or heat in vacuum to at least one facet forming the cavity. Thus a region having a band gap exceeding the effective band gap of the materials constituting the active layer is formed in the vicinities of the facet. That is to say, the region is made transparent to the emission wavelength of the semiconductor laser. Thus, high-performance semiconductor lasers with the window-structure capable of achieving both high output and high reliability can be easily obtained without suffering from the above problems encountering in the prior art. The present invention has been thus completed.
Accordingly, the gist of the present invention resides in a semiconductor laser having a first conduction type of clad layer, an active layer containing quantum well and a second conduction type of clad layer formed on a semiconductor substrate, characterized in that the active layer is made transparent to the emission wavelength by desorping a part of the constituting elements in the vicinities of at least one facet forming the cavity.
Another gist of the present invention resides in a process for producing a semiconductor laser having a first conduction type of clad layer, an active layer containing quantum well and a second conduction type of clad layer formed on a semiconductor substrate, characterized by comprising forming the first conduction type of clad layer, active layer containing quantum well and second conduction type of clad layer on the semiconductor substrate; and selectively desorping elements with high vapor pressure by irradiating in vacuum at least one facet forming the cavity with ion, electron, light and/or heat beam to thereby form a region which has been made transparent to the emission wavelength in the vicinities of the facet of the active layer.
The present inventors have further conducted extensive studies to solve the above-mentioned problem. As a result, they have found out that a compound semiconductor light emitting device having at least a first conduction type of clad layer, an active layer and a second conduction type of clad layer grown on a substrate, two facets which are opposite to each other forming a cavity and having an emission wavelength of xcex (nm), characterized in that the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming the transparent facets are coated with passivation layers, and the surfaces of the passivation layers are coated with a coating layer comprising a dielectric material optionally combined with a semiconductor, is much superior in high output and high reliability to the conventional ones, thus completing the present invention.
Accordingly, the gist of the present invention resides in a compound semiconductor light emitting device having at least a first conduction type of clad layer, an active layer and a second conduction type of clad layer grown on a substrate, two facets which are opposite to each other forming a cavity and having an emission wavelength of xcex (nm), characterized in that the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming the facets are coated with passivation layers made of Si, and the surfaces of the passivation layers are coated with a coating layer comprising a dielectric material optionally combined with a semiconductor.
A first aspect of the device is a compound semiconductor light emitting device of present invention, which comprises
a first conduction type of clad layer;
an active layer; and
a second conduction type of clad layer grown on a substrate and
two of said first conduction type of clad layer; said active layer, and said second conduction type of clad layer, being opposite to each other so as to form a cavity,
wherein said active layer is transparent to the emission wavelength in the vicinities of the facets and the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets are each coated with a passivation layer.
A second aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein at least one of the constituting elements of the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets exists in the form free from an oxide.
A third aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein the vicinities of said facets have been disordered.
A fourth aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein a coating layer comprising a dielectric material optionally combined with a semiconductor material is formed on the surface of said passivation layer.
A fifth aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein said passivation layer contains Si.
A sixth aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein one of said facets is coated with an anti-reflective coating layer containing an AlOx layer while the other is coated with a high-reflective coating layer containing AlOx layer and Si layer.
A seventh aspect of the device is a compound semiconductor light emitting device according to the first aspect, wherein said active layer comprises a compound semiconductor layer containing In.
An eighth aspect of the device is a compound semiconductor light emitting device of the present invention, which comprises:
a first conduction type of clad layer; an active layer; and a second conduction type of clad layer, grown on a substrate and
two facets of said first conduction type of clad layer; said active layer; and said second conduction type of clad layer, being opposite to each other so as to form a cavity,
wherein a conduction type of said active layer is p type and the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets are coated with a passivation layer containing Si.
A ninth aspect of the device is a compound semiconductor light emitting device according to the eighth aspect, wherein at least one of the constituting elements of the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets exists in the form free from an oxide.
A tenth aspect of the device is a compound semiconductor light emitting device according to the eighth aspect, wherein the vicinities of the facets of the cavity have been disordered.
A eleventh aspect of the device is a compound semiconductor light emitting device according to the eighth aspect, wherein said active layer comprises a compound semiconductor layer containing In.
A twelfth aspect of the method is a method of fabricating a compound semiconductor light emitting device of the present invention, which comprises the steps of:
growing a first conduction type of clad layer, an active layer and a second conduction type of clad layer on a substrate;
forming facets of a cavity; and
irradiating said facets of the cavity with plasma having energy of from 25 eV to 300 eV in vacuum.
A thirteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the twelfth aspect, wherein step of forming facets of a cavity comprises a cleavage of the first conduction type of clad layer, the active layer and the second conduction type of clad layer so that two facets are opposite to each other to form the cavity.
A fourteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the twelfth aspect, wherein plasma of an element of the group 18 is used in said irradiating step.
A fifteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the twelfth aspect, which further comprises a step of:
forming a passivation layer on each facet after said plasma irradiation step.
A sixteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the twelfth aspect, which further comprises a step of: forming an anti-reflective coating and/or a high-reflective coating on said facets while evacuating continuously after the irradiating step.
A seventeenth aspect of the method is a method of fabricating a compound semiconductor light emitting device of the present invention, which comprises the steps of:
growing a first conduction type of clad layer, an active layer and a second conduction type of clad layer on a substrate;
forming facets of a cavity by cleavage of the first conduction type of clad layer, the active layer and the second conduction type of clad layer so that two facets are opposite to each other so as to form a cavity;
removing, from said facets, a part of elements constituting the facet; and
forming a passivation layer on each facet after said removing step.
An eighteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the seventeenth aspect, wherein said removing step comprises a step of irradiating said facets with at least one selected from the group consisting of ion, electron, light and heat in vacuum.
A nineteenth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the seventeenth aspect, wherein plasma having energy of from 25 eV to 300 eV is used in said irradiating step.
A twentieth aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the seventeenth aspect, wherein plasma of an element of the group 18 is used in said irradiating step.
A twenty-first aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the seventeenth aspect, which further comprises a step of forming, on said passivation layer, a coating layer containing at least one combinations of dielectrics and semiconductors.
A twenty-second aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the seventeenth aspect, wherein said passivation layer contains Si.
A twenty-third aspect of the method is a method of fabricating a compound semiconductor light emitting device according to the twenty-first aspect, wherein, at the formation of said coating layer, the surface is irradiated with plasma simultaneously with the formation of the coating layer.